**Core Concept**
Heme synthesis is a complex biochemical pathway that involves the coordination of multiple enzymes and substrates to produce heme, a crucial component of hemoglobin, myoglobin, and other hemoproteins. The pathway begins with the synthesis of delta-aminolevulinic acid (ALA), which is then converted into porphobilinogen and eventually into heme.

**Why the Correct Answer is Right**
Glycine serves as the starting material in heme synthesis by donating a carbon atom to the synthesis of delta-aminolevulinic acid (ALA). This process occurs through the action of the enzyme ALA synthase, which catalyzes the condensation of glycine with succinyl CoA to form ALA. This reaction is a crucial step in the heme biosynthetic pathway and is essential for the production of heme.

**Why Each Wrong Option is Incorrect**
**Option A:** Lysine is not directly involved in heme synthesis and does not serve as a starting material for the production of delta-aminolevulinic acid (ALA).

**Option B:** Leucine is an essential amino acid involved in protein synthesis and energy metabolism, but it is not a precursor in the heme biosynthetic pathway.

**Option D:** Alanine is a non-essential amino acid involved in glucose metabolism and nitrogen transport, but it is not a starting material for heme synthesis.

**Clinical Pearl / High-Yield Fact**
Glycine is not only a starting material in heme synthesis but also plays a critical role in other biochemical pathways, including the synthesis of glutathione and purines. A deficiency in glycine can lead to impaired heme synthesis and porphyria, a group of disorders characterized by the accumulation of porphyrins in the body.

**✓ Correct Answer: C. Glycine**